US2363305A - Electronic control system - Google Patents

Description

Nov. 21, 1944. D. P. FAULK 2,363,305

ELECTRONIC CONTROL SYSTEM Filed 06.7"u 30, 1942 WITNESSES:

INVVENTOR 41 J DonaMPlZu/K, Z- B I AT.T EY

Patented Nov. 21, 1944 ELECTRONIC CONTROL SYSTEM Donald P. Faulk, Pittsburgh, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation Pennsylvania Application October 30, 1942, Serial No. 463,989

'7 Claims.

This invention relates to an electronic control system and has particular relation to a system in which power is supplied from an alternating potential source to a load through a pair of inversely connected electric discharge valves.

In many systems in which the supply of current to a load is controlled by inversely connected electric discharge valves, it is highly desirable to employ a single source of control potential for both valves. In a resistance welding system, for example. the fiow of welding current is supplied through a pair of inversely connected ignitrons. A firing circuit is provided for each ignitron and includes an electric discharge valve, preferably a thyration, connected between the anode and igniter of the corresponding ignitron, so that the firing valves themselves are inversely connected in circuit with the source. The firing valves are then controlled to render the ignitrons conductive alternately in successive half periods of the source. A control system is arranged to provide a pulsating control potential of suitable wave form for rendering the firing valves conductive at a selected instant in each half period for a predetermined interval of time. It is obviously desirable to employ a single control potential for both firing valves rather than provide a second control system. However, if a single source of control potential is to be connected inthe control circuit of both firing valves, the grids of the valves must be connected together and the oath odes must be connected together. To avoid a short circuit across the firing valves, a large impedance must be inserted in the connection be tween the cathodes. As a result, an alternating potentialap-pears across this impedance which interferes with the impressing of a control potential of suitable wave form between the grids and cathodes of firing valves. I

Different arrangements have been employed in the past in an attempt to counteract or nullify the alternating potential appearing across the impedance which is connected betweenthe cathodes of the firing valves. One way of accomplishing this result is to employ an inversely connected transformer to impress another alternating potential in the control circuit which is 180 out of phase with the source potential. However. in actual practice where current is conducted throughthe valves during only a portion of each half period, the alternating potential across the impedance between the cathodes is not a sine wave and consequently an alternating potential of sine wave form 180 out of phase with.

the source cannot effectively remove the undesired potential from the impedance.

Another method of counteracting the alter hating potential appearing across the impedance which is connected between the cathodes of the firing valves employs a pair of gaseous discharge rectifier tubes connected back-to-back across the impedance. The junction point between the two rectifiers is then connected to a center tap on the impedance, the rectifiers being arranged to conduct current away from the center tap. The control potential of suitable wave form is then impressed between the grids of the firing valves and the center tap on the impedance. In a halfperiod of one polarity, one of the rectifiers shortcircuits the one-half of the impedance in the control circuit of the firing valve which is in condition to becomeconductive. During the oppcsite half period, the other rectifier short-cir cuits the other half of the impedance which is in the control circuit of the other firing valve.

The prior art control arrangement just described operates satisfactorily on low voltage circuits. However, when a high voltage is employed, one rectifier becomes conductive just as the other has ceased to conduct current and the back potential appearing across the nonconducting rectifier causes it to backfire. As the conducting rectifier has extremely low resistance, a very large current passes through the backfiring rectifier and causes considerable damage thereto and disrupts the control of the firing valves.

It is, accordingly, an object of my invention to provide a new and improved system for supplying power from an alternating potential source to a load through a pair of inversely connected electric discharge valves in which a single control potential source is connected in circuit between the control electrode and cathode of both valves.

Another object of my invention is to provide a novel system for supplying power from an alternating potential source to a load through a pair of inversely connected electric discharge valves of the arc-like type in which a single source of periodically pulsating control potential is connected in circuit between the control electrode and cathodeof both valves.

More specifically, it is an object of my invention to provide anew and improved system for supplying power from an alternating potential source to a load through a pair of inversely connected electric discharge valves controlled by a single source of control potential in which the alternating potential is eifectively removed from the control circuit without danger of breakdown because of high voltage.

In accordance with my invention, a first, second and third impedance means is connected in series in the order named between the cathodes of the firing valves. A first gaseous rectifier discharge tube is connected to efiect short-circuiting of the first and second impedance means when the valve whose cathode is connected to the first impedance means has a positive anode potential. A second gaseous discharge rectifier tube is connected to short-circuit the second and third impedance means in the opposite half period of the source. The second impedance means may be short-circuited by either rectifier. The control potential is then impressed between the control electrode of the firing valves and a center tap on the second impedance means.

When one of the firing valves has a positive anode potential, one of the rectifiers becomes conductive to short-circuit the impedance in the control circuit of that valve. The rectifier does not, of course, have a zero arc drop potential. However, the rectifier arc. drop potential is so low that the voltage thereacross is very small, of'the order of 10 volts regardless of the magnitude of current thereto. Consequently a substantially constant residual potential of 10 volts appears across the two impedance means shorted by the rectifier but as the control potential is impressed between the :control electrode of the firing valve and the center tap of the second impedance means, the residual potential remaining in the control circuit is of the order of 6 volts, which is negligible for all practical purposes.

At the end of the first half-period the first rectifier ceases to conduct andthe other rectifier is rendered conductive to nullify the alternating potential in the control circuit of the second firing valve. It is to be noted that the second impedance means is connected between the two rectifiers. Then, if the first rectifier backfires, the current therethrough is limited to a low value because of the second impedance means in series therewith. Consequently, there is very little danger of permanent disruption of the control system by a backfiring of one of the rectifier tubes.

The novel features that I consider characteristic of my invention are set forth with particularity in the claims. The invention itself, however, both as to its organization and it method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accomof the source. The secondary H of the welding transformer i3 is connected across a pair of welding electrodes l9 and 2| in engagement with the material 23 to be welded. The firing circuit for one of the ignitrons I may be traced from the terminal I 5 of the source 3, through the anode 25 and cathode 21 of an electric discharge firing valve 29, the igniter 3| and cathode 33 of the ignitron and the primary H of the welding transformer l3 to the other terminal 5 01 the source. The firing circuit for the other ignitron 9 may be traced from the terminal 5, through the primary II, the anode 35 and cathode 31 of a second electric discharge firing valve 39, the igniter 4| and cathode 43 of the second ignitron 9 to the other terminal I5 of the source. It is thus apparent that the firing valves 29 and 39 are also connected inversely or in antiparallel in circuit with the source. The firing valves are of the arc-like type, preferably thyratrons, and their control electrodes 44 and 45 are interconnected through corresponding grid resistors 46 and 47 and a conductor 49. The cathodes 21 and 31 of the firing valves 29 and 39 are also interconnected through first, second and third resistors 5|, 52 and 53. A gaseous discharge rectifier tube 55 is connected across the first and second resistors 5| and 52 and a second gaseous discharge rectifier tube 51 is connected across the second and third resistors 52 and 53. The rectifier tube 55 is arranged to conduct current away from the junction point of the first and second resistors 5| and 52, and rectifier tube 51 is arranged to conduct current away from the junction point of the second and third resistor 52 and 53.

A control system 59 is provided to supply a control potential for controlling the firing valves 21 and 31. The control system includes a voltage divider 6| energized from the source 3 through an auxiliary transformer 63, a full wave rectifier 65 and filtering elements 51. An intermediate tap 69 on the divider 6| is connected to a center tap H I on the second resistor 52 interconnecting the cathodes of the firing valves 29 and 39. The negative terminal 13 of thedivider 6| is connected to the control electrodes 44 and 45 of the firing valves, The circuit from the negative terminal 13 of the divider El may be traced through resistors 15, 11 and 19, a balancing potentiometer circuit 8|, conductor 82, conductor 49 and resistor 46 to control electrode 44 or resistor 4| to control electrode'45. As will be explained hereinafter, the impressing of a control potential between the control electrodes 44 and'45 of the firing valves and the center tap H is the same as impressing the control potential directly between the control electrode and cathode of the firing valve whose anode is positive at the time.

If ignitrons and 9 are identical and firing valves 29 and 39 are identical, the balancing potentiometer circuit 8| does not supply a potential in the circuit. Therefore, function of the balancing potentiometer may be omitted from immediate consideration and will be explained hereinafter.

A potential is impressed across the resistor I9 in the circuit between the negative terminal 13 of the divider 6| and the control electrodes of the firin valves. This potential is derived from the source 3 through an auxiliary transformer 83 a phase shifting circuit 85 and a full wave rectifier 81, so that the potential takes the form of a rectified alternating potential, inverted with respect to the controlelectrodes and displaced in phase relative to the source.

A direct-current potential is impressed across the other resistor 1! in the circuit between the negative terminal 73 of the divider 6| and the' Portion of the divider 81 between the intermediate tap $9 and the negative terminal 13. Because of the high negative direct-current potential supplied from the divider SI and the resistor 11, the resultant potential impressed between the control electrodes of the firing valves 29 and 39 and the center tap of the second resistor 52 remains below the critical control electrode-cathode potential necessary to render the firing valves conductive.

Another electric discharge valve 93 of the arclike type, preferably a thyratron hereinafter designated as the start valve, is connected across the voltage divider GI in a circuit extending from the positive terminal 95 through a contactor 91 of a push-button switch 99, the anode IN and cathode I93 of the start valve 93 and parallel circuits comprising the resistor 15 on one side and a rectifler I05, potentiometer I01 and capacitor I99 on the other side to the negative terminal 13 of the divide: GI. The control circuit of the start valve 93 may be traced from its control electrode III, through a grid resistor H3, a resistor H5, and resistor 11 to the cathode I03. As previously set forth, a direct-current potential is impressed across the resistor 11 and this potential is of such polarity as to tend to maintain the start valve 93 nonconductive. Potential impulses are impressed across the other resistor I I5 in the control circuit of the start valve through a peaking transformer H1 and a phase shifting circuit II9 energized from the source. The phase shifting circuit H9 is arranged to be adjusted according to the power factor of the load so that an impulse across the resistor II5 occurs at the beginning of each half-period of alternating current. The polarity and magnitude of the impulse across the resistor II5 at the beginning of a positive halfperiod of the source current is such that the impulse overcomes the biasing potential across the resistor 11 and renders the start valve 93 conductive if the push-button contactor 91 is closed.

When the start valve 93 becomes conductive, the upper end of the resistor 15 in series therewith is effectively connected to the positive terminal 95 of the divider 6!. Consequently, the direct-current potential supplied from the divider to the circuit between the control electrodes of the firing valves and the center tap H of the second resistor 52 becomes considerably less negative. As a result, the peaks of the inverse rectified potential in the circuit rise above the critical control electrode-cathode potential necessary to render the firing valves conductive. This inverse rectified alternating potential is employed to determine the instant at which the firing valves 29 and 39 and, therefore, the ignitron 1 and 9 become conductive in a half-period of the source. The particular instant is selected by adjustment of the phase shifting circuit 85 to shift the position of the peaks and is effective to determine the amount of current supplied to the weld in a halfperiod, which, in turn, determines the character of the weld.

A second electric discharge valve I2l of the arc-like type, preferably a thyratron, which is hereinafter designated as a stop valve, is connected between the positive terminal 95 of the divider 6| and another intermediate tap I23 on the divider. The control circuit of the stop valve I2I may be traced from the control electrode I25 thereof through a grid resistor I21 and the capacitor I09 to the negative terminal 13 of the divider BI and from the intermediate tap I23 to the cathode I29 of the stop valve. The potenfirst firing valve 29.

tlal between the negative terminal 13 and intermediate tap I23 of the divider normally biases the stop valve to maintain it nonconductive. However, when the start valve 93 is conductive the capacitor I09 in series therewith is charged at a rate determined by the setting of the potentiometer I01. After a predetermined interval of time, the potential across the capacitor I09 rises to a suflicient magnitude to counteract the biasing potential in the control circuit of the stop valve I2I and render the stop valve conductive. When the stop valve I2I becomes conductive, the intermediate tap I23 is effectively connected to the positive terminal of the divider so that the upper end of resistor 15 is again highly negative with respect to intermediate tap 59. A back contactor I3I on the push-button switch 99 is arranged to complete a discharge circuit for the capacitor I09 when the push-button switch is released.

From the foregoing description it is apparent that the potential between the control electrodes of the firing valves 29 and 39 and the center tap H of the second resistor 52 between the cathodes 21 and 31 originally comprises an inverse rectified alternating potential superimposed on a highly negative direct-current potential of such value that the ignitrons are not rendered conductive. When the push-button contactor 91 is closed, the direct-current potential becomes less negative while the inverse rectified alternating potential remains the same and the ignitrons are rendered conductive alternately at-a selected instant in successive half-periods. After a preselected interval of time determined by the setting of the potentiometer I 01, thedirect-current potential again becomes highly negative and the flow of current through the ignitrons is halted.

A careful consideration of the control circuits .for the firing valves reveals that the impressing is positive at the time.

The three resistors 5|, 52 and 53 interconnecting the cathodes 21 and 31. of the firing valves 29 and 39 are also connected across the source 3 through the igniters 3| and M and cathodes 33 and 43 of the ignitrons 1 and 9. Consequently, an alternating potential in phase with the source appears across these resistors. When the potential across the resistors 5|, 52 and 53 is of such polarity that the cathode connected end of the first resistor 5] is negative, the first rectifier 55 becomes conductive to short-circuit the first and second resistors 5| and 52. It follows that the control potential impressed between the control electrodes 44 and 45 of the firing valves 29 and 39 and the center tap H of the second resistor 52 is then effectively impressed directly between the control electrode 44 and cathode E1 of the Similarly, when the cathode connected end of the third resistor 53 is negative, the second rectifier 51 becomes conductive to short-circuit the second and third resistors 52 and 53 and the control potential is effectively impressed between the control electrode 45 and cathode 31 of the second firing valve 39. Interference with the shape of the wave form of the control potential by the alternating potential across the three resistors is thus avoided. As a result, the control potential may be adjusted to rise above the critical value of the firing valves at any selected instant in a half-period and the ignitrons are rendered conductive at that selected instant.

Should the igniting characteristic of the two ignitrons be slightly difierent, the balance potentiometer 8! may be adjusted to add an alternating potential component to the control potential to raise alternate peaks of the inverse rectified alternating potential. As a result, the two ignitrons may be conductive for the same percentage of a half-period.

In a system connected as illustrated and tested, the ignitrons and 9 were Westinghouse WL-656 tubes, and the firing valves 29 and 39 were Westinghouse Wit-632 thyratrons. Each of the three resistors 52 and 53 had a resistance of 15,000 ohms and the gaseous discharge rectifiers 55 and 51 were Westinghouse WL-866A-866 tubes.

Although I have shown and described a preferred embodiment of my inventionQI am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted, except in so far as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. For use in supplying power from a source of alternating potential to a load, the combination comprising a pair of electric discharg valves, each having an anode and cathode connected between said source and load and a control electrode, said valves being connected in anti-parallel, a first, second and third impedance means connected in series in the order named between the cathodes of said valves whereby an alternating potential in phase with said source appears across said three impedance means, means responsive to a potential across said three impedance means of such polarity that the cathode connected end of said first impedance means is negative for short circuiting said first and second impedance means, means responsive to a potential across said three impedance means of opposite polarity for short circuiting said second and third impedance means, and means for impressing a control potential between said control electrodes and an intermediate tap on said second impedance means.

2. For use in supplying power from a source of means for short circuiting a second section of said impedance means, said first and second sections having a common portion with each section including part of said impedance means between said common portion and the cathode which is negative when the corresponding section is short circuited, and means for impressing a control p0- tential between said control electrodes and an intermediate tap on said common portion,

3. For use in supplying power from a source of alternating potential to a load, the combination comprising a pair of electric discharge valves, each having an anode and cathode connected between said source and load and a control electrode, said valvesbeing connected in anti-parallel, a first,

second and third impedance means connected in series in the order named between the cathodes of said valves, first rectifying means connected across said first and second impedance means in a manner to conduct current away from the junction of said second and third impedance means, second rectifying means connected across said second and third impedance means in a manner to conduct current away from the junction of said first and second impedance means, and means for impressing a control potential between said control electrodes and an intermediate tap on said second impedance means.

4. For use in supplying power from a source of alternating potential to a load, the combination comprising a pair of electric discharge valves, each having an anode and cathode connected between said source and load and a control electrode, said valves being connected in anti-parallel, a first, second and third impedance means connected in series in the order named between the cathodes of said valves, first rectifying means of the gaseous discharge type connected across said first and second impedance means in a manner to conduct current away from the junction of said second and third impedance means, second rectifying means of the gaseous discharge type connected across said second and third impedance means in a manner to conduct current away from the junction of said first and second impedance means, and means for impressing a control potential between said control electrodes and an intermediate tap on said second impedance means.

5. For use in supplying power from a source of alternating potential to a load, the combination comprising a pair of electric discharge valves of 1 the arc-like type, each having an anode and cathode connected between said source and load and a control electrode, said valves being connected in anti-parallel, a first, second and third impedance means connected in series in the order named between the cathodes of said valves, first rectifying means connected across said first and second impedance means in a manner to conduct current away from the junction of said second and third impedance means, second rectifying means connected across said second and third impedance means in a, manner to conduct current away from the junction of said first and second impedance means, and means for impressing a periodically pulsating control potential having a selected phase relation to said source between said control electrodes and an intermediate tap on said second impedance means.

6. For use in supplying power from a source of alternating potential to a load, the combination comprising a pair of electric discharge valves of the arc-like type, each having an anode and cathode connected between said source and load and a control electrode, said valves being connected in anti-parallel, impedance means connected between the cathodes of said valves whereby an alternating potential in phase with said source appears across said impedance means, means responsive to a potential of one polarity across said impedance means for short circuiting a first section of said impedance means, means responsive to a potential of opposite polarity across said impedance means for short circuiting a second section of said impedance means, said first and second sections having a common portion with each section including part of said impedance means between said common portion and the cathode which is negative when the corresponding section is short circuited, and means for impressing a periodically pulsating control potential having a selected phase relation to said source between said control electrodes and an intermediate tap on said common portion.

"7. For use in supplying power from a source of alternating potential to a load, the combination comprising a pair of electric discharge valves of the arc-like type, each having an anode and cathode connected between said source and load and a control electrode, said valves being connected in anti-parallel, a first, second and third resistance means connected in series in the order named between the cathodes of said valves, first rectifying means of the gaseous discharge type DONALD P. FAULK.

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